The invention relates to the manufacture of a partially permeable membrane from an aluminum foil using electrolytic etching and relates also to the use of this membrane.
Partially permeable membranes are employed in a series of separation processes in which particles dispersed or dissolved in a liquid or gaseous medium have to be separated or in which gases have to be separated.
The membranes normally employed for that purpose are based on polymer foils and hollow polymer fibers. These, however, give rise to some difficulties which have serious consequences in certain applications: limited thermal stability (to some extent prohibits sterilization); limited stability in organic solvents, swelling in water, usually no electrical conductivity (for sensor applications).
Some help here is a process for manufacturing membranes having pore diameter 0.2 to 10 .mu.m out of aluminum in which a recrystallized aluminum foil is electrolytically etched. In that case a first etching stage is employed with a constant anode potential, throughout the duration of the stage and above the pitting potential of the aluminum, and a second etching stage is employed with a constant anode potential below the pitting potential. A proposal for narrowing the pores further is made in which an aluminum oxide layer is anodically formed on the etched foils or a hydrated aluminum oxide layer is formed by treatment in boiling water, as a result of which pore diameters down to 0.002 .mu.m can be achieved.
Aluminum membranes manufactured that way, however, suffer decisive disadvantages:
Potentiostatic etching is, in practice, very difficult to control. There is in particular the danger that the smallest deviation in the effective potential from the optimal value leads to an undesired micro-pitting which excludes the product from use as a membrane foil. Even more decisive is the fact, that the membranes manufactured by the above mentioned process are unstable as soon as they are employed for the filtration of aqueous media or non-aqueous media containing the smallest amount of water. This instability is expressed in a rapid drop in trans-membrane flow, which after a few hours is typically only 5 to 10% of the initial value and thus insufficient and uninteresting for microfiltration membranes compared to conventional polymer membrane foils. This drop is observed both with foils etched on one side and foils etched on both sides, with blank foils and with foils having an anodic aluminum oxide barrier layer.